1. Field of the Invention
The present invention relates to a device for coding a picture signal by compression and, more particularly, to a picture signal compression coding device which maintains the amount of compressed data representative of a picture constant at all times.
2. Description of the Background Art
Digital picture data representative of a picture picked up by an electronic still camera, for example, are stored in a memory. Various kinds of compression coding schemes have been proposed to reduce the amount of such digital picture data and thereby the required memory capacity. Among them, a two-dimensional orthogonal transform coding scheme is extensively used because it codes data by a large compression ratio and because it allows a minimum of picture distortions particular to coding to occur.
Two-dimensional orthogonal transform coding is such that picture data representative of a single picture are divided into a plurality of blocks, and the picture data are subjected to two-dimensional orthogonal transformation block by block. The picture data undergoes orthogonal transformation, i.e., transform coefficients are compared with a predetermined threshold so as to discard those transform coefficients which are lower than the threshold. The transform coefficients lower than the threshold are treated as data zero thereafter. The remaining data are divided by a predetermined quantizing step value, or normalizing coefficient, and thereby quantized or normalized by a step size. By this kind of procedure, the values of transform coefficients, i.e., the dynamic range of amplitudes, is suppressed.
The comparison of transform coefficients with a threshold and the normalization are often executed at the same time. Specifically, when transform coefficients are normalized by a predetermined normalizing coefficient and the results are turned to integers, the transform coefficients which are smaller than the normalizing coefficient turn out to be zero. The normalized transform coefficients are Huffman-coded and then stored in a memory.
In such a two-dimensional orthogonal transformation coding procedure, coding picture data by applying a predetermined normalizing coefficient to transform coefficients is problematic in that since the amount of data differs from one coded picture data to another, troublesome processing is needed in writing the picture data to in a memory.
In the light of the above, there has been proposed a system which calculates the degree to which high frequency components are predominant, i.e., an activity is calulated block by block and a normalizing coefficient is selected in matching relation to the calculated activity. By so determining the normalizing coefficient, it is possible to maintain the amount of data representative of a picture constant.
Assume that the number of bits of coded data to be assigned to a single picture is constant. Then, since the individual blocks constituting the picture have different activities, it is desirable that a different number of bits be allocated to each block commensurately with the activity. A prerequisite with such a manner of bit allocation is that the activities of the individual blocks be calculated with accuracy.
Concerning the calculation of block-by-block activities, there has been proposed a method which consists of subdividing a block into a plurality of subblocks, determining the activity subblock by subblock, and adding the activities of the individual subblocks to produce the activity of the entire block. This kind of procedure is successful in promoting the accuracy of block-by-block activity calculation. However, when the pixel data in a certain subblock forming a part of a block have the same level, for example, the calculated activity of the block will be zero even if the subblock of interest is different from the others with respect to the level of the pixel data. The prior art subblock scheme, therefore, is not fully satisfactory when it comes to accurate calculation of block-by-block activities.